Charged particle beam instrument

ABSTRACT

A charged particle beam instrument capable of reducing the spread of the probe diameter while maintaining the probe current constant. An electrical current I d  is detected by a detection aperture to create a feedback signal. The feedback signal is supplied to a condenser lens control and to an objective lens control via a signal adjuster. The objective lens control portion controls the objective lens such that the charged particle probe is in focus.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a charged particle beaminstrument, such as an electron probe microanalyzer or a scanningelectron microscope.

[0003] 2. Description of the Related Art

[0004] In a charged particle beam instrument, such as an electron probemicroanalyzer or a scanning electron microscope, a charged particle beamemitted from a charged particle beam source is accelerated and focusedonto a specimen by a condenser lens system and an objective lens. As thecharged particle beam hits the specimen, X-rays and secondary particlesare produced, and these are detected.

[0005] In this kind of instrument, the current of the charged particleprobe made to hit the specimen is stabilized. FIG. 1 is a diagramschematically illustrating this probe current-stabilizing function. Acharged particle beam CB produced by a charged particle beam source (notshown) and accelerated is sharply focused onto a specimen 3 by acondenser lens system 1 and an objective lens 2.

[0006] A detection aperture 4 is located between the condenser lenssystem 1 and the objective lens 2 and detects an outer portion of thecharged particle beam. The output signal from the detection aperture 4is amplified by a feedback device 5 and supplied to a control portion 6for the condenser lens system 1 for adjusting the probe current.

[0007] The control portion 6 adjusts the strength of the condenser lenssystem 1 according to the magnitude of a reference signal and themagnitude of the output signal from the feedback device 5. A feedbackloop is formed in this way. Therefore, the current of the chargedparticle beam probe P impinging on the specimen 3 can be kept constantin principle if the current density of the charged particle beam doesnot vary.

[0008] To establish negative feedback (i.e., to prevent positivefeedback as described in Japanese Patent Laid-Open No. 183044/1989), anaperture for limiting peripheral portions of a charged particle beamexiting from the condenser lens system is placed ahead of the detectionaperture 4 as described in Japanese Technical Review 82-7798. Thisaperture is omitted in FIG. 1.

[0009] The detection aperture 4 can also be designed to act also as anobjective aperture for controlling the probe current and the divergenceangle of the probe.

[0010] As mentioned previously, where negative feedback is applied tothe condenser lens system 1, if the exciting current supplied to thecondenser lens system 1 is varied so as not to vary the probe current,the position of the focal point of the condenser lens system 1automatically changes from the state indicated by the solid line to thestate indicated by the broken line. It is now assumed that some changeoccurs in the charged particle beam source and that the probe currentshould vary from I_(p) by ΔI_(p). However, the negative feedback variesthe distance between the detection aperture 4 and the focal point, thusmaintaining the probe current I_(p) constant.

[0011] In spite of this, an adjustment of the condenser lens system 1moves the focal position of the charged particle probe P on the specimenout of the specimen surface by Δb. The spread portion Δd_(1p) of theprobe diameter due to the feedback adds to the final probe diameterd_(p).

[0012] It is assumed that the objective lens 2 has an object distance ofa and an image distance of b. If the focal distance f_(OL) of theobjective lens 2 is constant, the following relation holds:

db/da=−M²

[0013] where M (=b/a) is the magnification of the objective lens.Therefore, when the object distance varies by a small distance of Δa,the image distance deviates by Δb, which is given by:

Δb=−M2.Δa

[0014] That is, the image distance deviation Δb can be reduced bycombining the lenses so as to reduce the magnification M (=b/a). It canbe seen, however, that the deviation Δb cannot be reduced to any desiredsmall value, because the number of lenses is finite, and because themicroscope column has a finite length.

[0015] On the other hand, in an instrument equipped with a chargedparticle beam source of low brightness, the final probe diameter d_(p)is not thin. Therefore, the spread Δd_(1p) of the probe diameter due tonegative feedback presents no serious problems. In contrast, emission ofa charged particle beam from a charged particle beam source of highbrightness (e.g., field emission, electron emission, such as Schottkyemission, and ion emission due to field ionization or electrolyticdissociation) can produce a quite thin final probe diameter d_(p).Consequently, the spread Δd_(1p) of the probe diameter due to negativefeedback can no longer be neglected.

[0016] Furthermore, in a charged particle beam source of highbrightness, the emission current tends to vary. This increases theamount of correction made by negative feedback. This, in turn, increasesthe spread Δd_(1p) of the probe diameter, thus increasing the amount ofdefocus.

[0017] A charged particle beam source of high brightness is adopted toobtain a small probe diameter. This object cannot be achieved due to thespread Δd_(1p) of the probe diameter, which, in turn, is caused bynegative feedback that is used to obtain a stable probe current.

SUMMARY OF THE INVENTION

[0018] It is an object of the present invention to provide a chargedparticle beam instrument capable of reducing the spread of the probediameter greatly while maintaining the probe current constant.

[0019] This object is achieved by a charged particle beam instrumentthat has a charged particle beam source for producing a charged particlebeam having a probe current, a first focusing means for focusing thecharged particle beam and varying the probe current of the chargedparticle beam impinging on a specimen, a second focusing means forvarying the degree of focus of the charged particle beam impinging onthe specimen, a first control portion for controlling the first focusingmeans, and a second control portion for controlling the second focusingmeans. This charged particle beam instrument is characterized in that itis equipped with a means for detecting a part of the current of thecharged particle beam from the charged particle beam source to therebyproduce a detected signal, controlling the control portion for the firstfocusing means to maintain constant the probe current of the chargedparticle beam impinging on the specimen, and controlling the controlportion for the second focusing means according to the detected signalto adjust the focus of the charged particle beam.

[0020] Other objects and features of the invention will appear in thecourse of the description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a diagram illustrating the prior art charged particlebeam instrument;

[0022]FIG. 2 is a diagram illustrating the fundamental structure of acharged particle beam instrument in accordance with the presentinvention; and

[0023]FIG. 3 is a diagram showing one specific example of a feedbacksignal-processing circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024] An embodiment of the present invention is hereinafter describedin detail by referring to the accompanying drawings. FIG. 2 shows thefundamental structure of a charged particle beam instrument inaccordance with the present invention. Like components are indicated bylike reference numerals in various figures including FIG. 1 used todescribe the prior art structure. Those components which have beenalready described will not be described below.

[0025] Referring to FIG. 2, a detection aperture 4 detects an electricalcurrent I_(d). A feedback device 5 converts the detected current I_(d)into a voltage and creates a signal for feedback (referred to as the“feedback signal” herein). The output signal from the feedback device 5is fed to a condenser lens control portion 6 and to an objective lenscontrol portion 8 via a signal adjuster 7. The objective lens controlportion 8 compares the feedback signal whose amplitude is adjusted bythe signal adjuster 7 with an objective lens control signal and controlsthe strength of the objective lens 2 according to the result of thecomparison.

[0026] To establish this negative feedback, an aperture (not shown inFIG. 2) is mounted between the condenser lens system 1 and the detectionaperture 4 to limit peripheral portions of the charged particle beamexiting from the condenser lens system 1.

[0027] The operation of the instrument of the construction describedthus far is next described. Let V_(CL) (=V_(CLO)) be an input signalapplied to the condenser lens control portion 6 before applying negativefeedback to it. Let V_(OL)(=V_(OLO)) be an input signal applied to theobjective lens control portion 8. The following signals corresponding tothese input signals are applied to the condenser lens system 1 and theobjective lens 2, respectively.

[0028] I_(CL)=ρ_(CL).V_(CL)

[0029] I_(OL)=ρ_(OL).V_(OL)

[0030] where ρ_(CL) and ρ_(OL) are constants. Where the lenses are ofthe electrostatic type, I_(CL) and I_(OL) correspond to voltages. Wherethe lenses are of the magnetic type, they correspond to excitingcurrents.

[0031] Negative feedback is applied by the detection aperture 4, thefeedback device 5, the condenser lens control portion 6, and thecondenser lens system 1 such that the current I_(d) detected by thedetection aperture I_(d) is kept substantially constant if the detectedcurrent I_(d) increases monotonously when the probe current I_(p)increases. The conditions under which negative feedback is establishedare described, for example, in the above-cited Japanese Patent Laid-OpenNo. 183044/1989. The operation is described in further detail.

[0032] First, a state in which negative feedback is not applied isdiscussed. It is assumed that some change has occurred in a particlesource, resulting in the following changes:

I^(pO)→I_(pO)+ΔI_(p)

I_(d0)→I_(d0)+ΔI_(d)

[0033] It is first assumed that a signal ΔV_(CL) is added to thecondenser lens control portion 6 to return the probe current to itsoriginal value I_(pO) and that the position of the focal point of thecondenser lens system 1 has been varied thereby. Then, a signal ΔV_(OL)is applied to the objective lens control portion 8 to prevent the degreeof focus of the particle probe from being varied by the change Δa in theobject distance. These signals ΔV_(CL) and ΔV_(OL) should be applied tothe condenser lens control portion 6 and the objective lens controlportion 8, respectively, by some method to maintain constant the degreeof focus of the particle probe while maintaining constant the probecurrent and the detected current.

[0034] A state in which negative feedback is applied is next discussed.Under the presence of negative feedback, some cause on the side of thecharged particle beam attempts to vary the probe current I_(p) and thedetected current I_(d). However, these are kept almost constant becauseof negative feedback applied by the feedback device 5 to the condenserlens control portion 6.

[0035] If the probe current I_(p) and the detected current I_(d) arekept constant, the signal applied to the condenser lens control portion6 from the feedback device 5 should be equal to the above-describedΔV_(CL), as can be seen from the description provided above. To obtainnormal negative feedback operation, the amplification degree of thefeedback device 5 with respect to the signal from the detection aperture4 is designed to have a sufficiently large value. Therefore, this signalΔV_(CL) is created.

[0036] On the other hand, the signal created by negative feedback isΔV_(CL). This signal is amplified or attenuated, and the signal adjuster7 is so operated that the resulting signal is equal to theaforementioned signal ΔV_(OL). The operation of the signal adjuster 7may be determined according to the state of the operating chargedparticle beam instrument (e.g., the energy E of the particle beam, theprobe current I_(p), and the image distance b of the objective lens 2).

[0037] The signal adjuster 7 may be so designed that its output is inproportion to the input signal ΔV_(CL) (linear output). If necessary,the following nonlinear calculations may be involved.

[0038] (ΔV_(CL))^(n)(n=0, 1, 2, . . . )

[0039] sin (n.k ΔV_(CL)) (n=0, 1, 2, . . . ; k is a constant)

[0040] The general fundamental structure and the principle of operationhave been described thus far. A more specific example is described belowby referring to FIG. 3, which shows a modification of the feedbackdevice 5 and a modification of the signal adjuster 7, it being notedthat the feedback device 5 and the signal adjuster 7 are shown in FIG.2.

[0041] In the configuration of FIG. 3, if an instruction for start ofstabilization of the probe current is given from the outside, thecurrent I_(d)=I_(dO) of the charged particle beam detected by thedetection aperture 4 is converted into a signal voltage by acurrent-to-voltage converter 11 immediately before negative feedback isapplied to the condenser lens system 1.

[0042] The output signal V_(1d)=V_(1d0) from the current-to-voltageconverter 11 is applied to an A/D converter 12 that converts an analogsignal into a digital signal. The A/D converter 12 sends data AD_(1d0)corresponding to the V_(1d0) to the control portion 13. The controlportion 13 saves this data and sends data DA_(1d0) to a D/A converter 14that converts a digital signal to an analog signal.

[0043] The output from the D/A converter 14 remains the same as theoutput V_(1d0) from the current-to-voltage converter 11 immediatelybefore application of negative feedback until a next instruction forstart of stabilization of the probe current is given. Then, the outputV_(1d) from the current-to-voltage converter 11 and the output V_(1d0)from the D/A converter 14 are applied to an adder 15, which, in turn,produces the difference ΔV_(1d)(=V_(1d)−V_(1d0)) between them.

[0044] The output from the adder 15 is applied to an amplifier 16 whosegain can be set to a sufficiently large value A1. Immediately beforeapplication of negative feedback, the difference ΔV_(1d)=0. Whennegative feedback is subsequently applied to the condenser lens system 1in practice, a switch 19 is turned on. Because of the setting to thesufficiently large gain A1, output ΔV_(CL)=A1.ΔV_(1d) is delivered.

[0045] It is obvious that ΔV_(CL)=0 holds immediately after applicationof negative feedback. Then, a switch 20 is turned on. The signal isapplied to an amplifier 17 whose gain can be set to A2. The output fromthe amplifier 17 is applied to a D/A converter 18. The output from theamplifier 16 is multiplied by a factor of A2.r2, where r2 is a signalratio indicated by data DA2 from the D/A converter 18. The followingsignal is delivered from the D/A converter 18:

ΔV _(OL) =A2.r2.ΔV _(CL)

[0046] The signal ΔV_(CL) obtained in this way is added to the condenserlens control portion 6, while ΔV_(OL) is applied to the objective lenscontrol portion 8.

[0047] If some change occurs in the charged particle beam source, and ifthe probe current is not stabilized, the probe current I_(p) and thedetected current I_(d) should vary as follows. However, because of theaction of negative feedback applied to the condenser lens system 1, thecurrents I_(p) and I_(d) can be kept substantially constant.

[0048] I_(p0)→I_(p0)+ΔI_(p)

[0049] I_(d0)→I_(d0)+ΔI_(d)

[0050] The relation of the objective lens current signal V_(OL), whichmaintains the degree of focus of the particle probe, to the condenserlens control current V_(CL) is given by:

V_(OL)=F (V_(CL))

[0051] We have:

dV _(OL) /dV _(CL) =F′(V _(CL))

[0052] A variation for maintaining the probe current constant is givenby:

V _(CLO) →V _(CL0) +ΔV _(CL)

[0053] A variation for maintaining the degree of focus for the signalvariation ΔV_(CL) is given by:

V _(OLO) →V _(CLO) +ΔV _(OL)

[0054] If the variation ΔV_(CL) in V_(CL) is infinitesimal, thevariation ΔV_(OL) in ΔV_(OL) can be found, using the aforementioneddifferential coefficient F′(V_(CL)), from:

ΔV _(OL) =F′(V _(CLO))ΔV _(CL)

[0055] That is, the variation ΔV_(OL) is in proportion to the variationΔV_(CL) if the signal ΔV_(CL) is small. Accordingly, if the instrumentis so set up that the gain A2 of the amplifier 17 and the signal ratior2 of the D/A converter 18 satisfy the relation:

A2.r2=F′(V _(CLO))

[0056] then the degree to which the particle probe is focused can bekept constant by the use of the signal ΔV_(CL) that is employed tostabilize the probe current.

[0057] While one embodiment of the present invention has been describedthus far, the invention is not limited to this embodiment. Rather,various changes and modifications are possible. For example, in theabove embodiment, a part of the charged particle beam is used as adetected signal. If the degree to which the particle probe is focused isvaried by negative feedback other than the negative feedback using thedetection aperture 4, the signal for the negative feedback may beadjusted, and this adjusted signal may be applied to the control portionfor the condenser lens system and to the control portion for theobjective lens.

[0058] As an example, a variation Ale in the emission current I_(e) in aparticle beam emission source is detected, and negative feedback isapplied to the extraction voltage V_(ex). In this case, the varyingsignal I_(e) is adjusted and used.

[0059] In the above-described embodiment, a signal for adjusting thefocal distance of the objective lens is added to the lens. It is to benoted that the invention is not limited to this scheme. For example, asignal for adjusting the focal distance of the lens may be added to acontrol portion for a focus-adjusting auxiliary lens located between thedetection aperture 4 and the objective lens 8 or to a control portionfor a control lens for adjusting the aperture angle of the beam incidenton a specimen. Also, in this case, the object of the present inventioncan be accomplished. That is, a slight amount of defocus affects thefinal probe diameter greatly. In contrast, a slight deviation from theoptimum aperture angle does not affect the final probe diameter.

[0060] The configuration shown in FIG. 3 can be applied to the systemshown in FIG. 5 of the above-cited Japanese Patent Laid-Open No.183044/1989. An example of its application is now described. X-axisdetection electrodes arranged symmetrically around a charged particlebeam produce output currents I_(X1) and I_(X2), respectively. Y-axisdetection electrodes perpendicular to the X-axis detection electrodesproduce output currents I_(Y1) and I_(Y2), respectively. Amounts ofsignals indicating the amounts of shifts of the charged particle beamfrom the optical axis in the X- and Y-axes are given by:

V _(X)=(X ₁ −X ₂)/(X ₁ +X ₂)

V _(Y)=(Y ₁ −Y ₂)/(Y ₁ +Y ₂)

[0061] These amounts of signals are calculated by an arithmetic unit orthe like. An amount of signal corresponding to the magnitude of the beamcurrent is given by:

V _(T) =X ₁ +X ₂ +Y ₁ +Y ₂

[0062] Then, these signals of these amounts are converted into digitalsignals immediately before start of application of negative feedback.The obtained signals are converted into analog signals to find referencesignals V_(X0), V_(Y0), and V_(T0). Signals V_(X), V_(Y), and V_(T) aredetected immediately after start of application of negative feedback.The differences between these signals V_(X), V_(Y), and V_(T) and thereference signals V_(X0), V_(Y0), and V_(T0) are given by:

ΔV _(X) =V _(X) −V _(X0)

ΔV _(Y) =V _(Y) −V _(Y0)

ΔV _(T) =V _(T) −V _(T0)

[0063] These differential signals are found.

[0064] Finally, these are amplified by a sufficiently large factor andused as signals for negative feedback. That is, ΔV_(X) and ΔV_(Y) areused as signals for correcting beam shifts in the X- and Y-directions,respectively. ΔV_(T) is used as a signal for correcting the probecurrent and as a signal for correcting defocus where the probe currentis corrected in this way.

[0065] In this example of application and in the case of FIG. 3,negative feedback is described using symbols of digital switches thatare opened and closed. Instead, analog switches may be used. That is, asignal for negative feedback may be applied gradually. Similarly, anamplifier whose gain is increased gradually may be used. In this case,even if the differential signals ΔV_(1d), ΔV_(X), ΔV_(Y), and ΔV_(T) arefinally amplified with extremely large degrees of amplification, thesignal system will not be saturated. Furthermore, stable negativefeedback is possible.

[0066] As described thus far, in the present invention, a part of acharged particle beam is detected. In response to the detected signal, afirst lens is controlled to maintain constant the current of the chargedparticle beam made to hit a specimen. In response to the detectedsignal, a second lens is controlled to adjust the focus of the chargedparticle beam. Therefore, the current of the charged particle probedirected to the specimen can be kept constant at all times withoutdefocus.

[0067] In another embodiment of the invention, a signal ΔV_(CL) usedwhen negative feedback is applied to the control portion for the firstlens according to the detected signal is produced by amplifying thedifference between V_(1d) and V_(1d0) (ΔV_(1d)=V_(1d)−V_(1d0)) whilemaintaining the signal V_(1d0) corresponding to the current detectedimmediately before application of negative feedback. The signal V_(1d)corresponds to the current detected after the start of negativefeedback. A signal ΔV_(OL) proportional to the feedback signal ΔV_(CL)supplied to the control portion for the first lens is fed to the controlportion for the second lens. Consequently, stabilization of the focus ofthe charged particle probe can be accomplished with a simple structure.When negative feedback is started, the signal for feedback starts at 0.Therefore, the dynamic range of the signal for feedback can be madewide.

[0068] Having thus described my invention with the detail andparticularity required by the Patent Laws, what is desired protected byLetters Patent is set forth in the following claims.

What is claimed is:
 1. A charged particle beam instrument comprising: acharged particle beam source for producing a charged particle beamhaving a probe current; a first focusing means for focusing the chargedparticle beam and varying the probe current of the charged particle beamimpinging on a specimen; a second focusing means for varying the degreeof focus of said charged particle beam impinging on the specimen; afirst control portion for controlling said first focusing means; meansfor detecting a part of the current of the charged particle beam fromthe charged particle beam source to thereby produce a detected signal,controlling the control portion for said first focusing means tomaintain constant the probe current of the charged particle beamimpinging on the specimen, and controlling the control portion for saidsecond focusing means according to the detected signal to adjust focusof said charged particle beam.
 2. The charged particle beam instrumentof claim 1 , wherein said second focusing means is an objective lensthat adjusts strength of an objective lens for adjusting the focus ofsaid charged particle beam.
 3. The charged particle beam instrument ofclaim 1 , wherein said second focusing means is an auxiliary lenslocated close to an objective lens, and wherein strength of saidauxiliary lens is adjusted to adjust the focus of said charged particlebeam.
 4. The charged particle beam instrument of claim 1 , wherein anaperture is positioned between said first focusing means and said secondfocusing means to detect the charged particle beam incident on saidaperture.
 5. The charged particle beam instrument of claim 1 , wherein asignal ΔV_(CL) used when feedback is applied to the control portion forsaid first focusing means according to said detected signal is obtainedby amplifying the difference ΔV_(1d) (=V_(1d)−V_(1d0)) between a signalV_(1d) corresponding to a current detected after start of the feedbackand a signal V_(1d0) corresponding to a current detected immediatelybefore the application of the feedback while maintaining the signalV_(1d0) corresponding to the signal detected immediately before theapplication of the feedback.
 6. The charged particle beam instrument ofclaim 5 , wherein a signal ΔV_(OL) proportional to the feedback signalΔV_(CL) supplied to the control portion for said first focusing means issupplied to the control portion for said second focusing means.